78 Roper, on the Diatomaceoe of the Thames. 
mouths of large and slowly-flowing rivers, such, for instance, 
as the Mississippi, in which the mean velocity of the current 
at New Orleans is only about one mile and a half per hour 
for the whole body of water. Sir Charles Lyell, from expe- 
riments on the proportion of sediment carried down by the 
river, has calculated that, taking the area of the delta at 13,600 
square miles, and the quantity of solid matter brought down 
annually at 3,702,758,400 cubic feet, it must have taken 67,000 
years for the formation of the whole.* Now, as the siliceous 
frustules of the Diatomacece are secreted from the water alone, 
and would most probably be extremely abundant in so 
sluggish a stream (especially as Professor Bailey has found 
both marine and fresh-water species abundant in the rice- 
grounds), there can be little doubt that, without taking the 
larger proportion noticed by Ehrenberg in the Elbe, even if it 
were considerably less, it would reduce the above period by 
several thousand years, and the same cause would probably 
apply with equal force to the Ganges and Nile. M. Ehren- 
berg considered that at Pillau there are annually deposited 
from the water from 7,200 to 14,000 cubic metres of fine 
microscopic organisms, which in the course of a century 
would give a deposit of from 720,000 to 1,400,000 cubic 
metres of infusory rock or Tripoli stone. 
My principal object in the foregoing paper has been to 
direct the attention of microscopists more particularly to the 
Diatomacece deposited by rivers and in tidal harbours, not only 
in those localities where they occur in overwhelming abun- 
dance, on the surface of quiet estuary waters, but in the mud 
itself, in which many of the rarer forms, and doubtless many 
new species, are yet to be found. That such an examination 
is still a desideratum is, I think, shown by the fact that out of 
the 279 species described by Mr. Smith in the first volume of 
his 'Synopsis,' only six are given as inhabitants of the 
Thames, and a very limited number to the Avon, Orwell, and 
some other rivers ; whilst the Severn, the Mersey, and many 
of our tidal harbours are altogether unnoticed. 
That examinations of this nature may sometimes prove 
useful in an economical point of view is very probable, parti- 
cularly as it has been noticed that the best samples of guano 
contain the greatest number of these siliceous skeletons, which 
doubtless serve to replace the large amount of silica ab- 
stracted from the soil by the cereal crops. Hence it is pro- 
bable that the deposits of many of our rivers would have a 
beneficial effect if applied to the land, and it rests with the 
microscopist to point out the most favourable localities for 
* Lyell's Principles of Geology, 8th edit., p. 219. 
